Both the Long Term Evolution (LTE) and Wideband Code Division Multiple Access (WCDMA) radio communication standards use persistent protocols, where Radio Link Control (RLC) is the key protocol to secure that all Service Data Units (SDUs) are correctly delivered and in the correct order (in-sequence-delivery). RLC, among other protocols, includes a retransmission functionality to support this. To be able to fulfil in-sequence-delivery requirements, any lost RLC Protocol Data Unit (PDU) must be retransmitted. Meanwhile on the receiver side, there is no data delivered from the RLC layer to higher layers until the lost PDU is correctly received. A rough estimation gives 0.5% RLC retransmission rate for Enhanced Uplink (EUL), 2 & 10 milliseconds (ms), and even higher for R99 uplink (R99 is the standard defined in the Third Generation Partnership Project (3GPP) Release '99, the original Universal Mobile Telecommunications System (UMTS) release).
Later trends have been towards the direction of increased Hybrid Automatic Repeat Request (hybrid ARQ or HARQ) failure rate and RLC Block Error Rate (BLER) to improve the cell capacity of cellular communication networks.
FIG. 1 illustrates the transmission, in accordance with prior art, of two higher layer SDUs, 1st SDU and 2nd SDU, from a User Equipment (UE) to a service provider via a Radio Access Network (RAN) node. The 1st SDU and the 2nd SDU are segmented into a 1st PDU and a 2nd PDU, respectively (here for simplification assuming that there is a 1:1 relationship between PDUs and SDUs). The RAN node could e.g. be a Radio Network Controller (RNC) or an Evolved Node B (eNB). The 2nd SDU contains data which is insensitive to out-of-sequence delivery. The 1st PDU and the 2nd PDU are transmitted from the UE. The RAN node receives the 2nd PDU and detects that the 1st PDU is missing (there is a gap in the PDU sequence). The RAN node thus requests retransmission of the 1st PDU. When the RAN node has successfully received the 1st PDU, the 1st SDU and the 2nd SDU are sent to the service provider.
Network nodes (for example routers, switches, RNC, Radio Base Station (RBS) e.g. eNB, “sub-nodes” etc.) have buffers to handle data bursts without packet loss. The buffers are dimensioned based on performance, memory, design competence etc. This means that especially older versions of nodes like routers and RNC may have a somewhat poor buffer dimensioning in some scenarios. Poor buffer dimensioning leads to packet loss for larger bursts. The packet losses from bursts cause Transmission Control Protocol (TCP) congestion control to back-off, and this eventually leads to reduced end user performance.
A problem is that an RLC retransmission will cause an outage/delay in the data transfer to the end point (e.g. a higher layer or another node).
Another problem is that when the lost RLC PDU is correctly received, all correctly and in-sequence-delivered PDUs can be delivered to higher layers (in the form of derived SDUs). This will create a larger burst of data, which can be a problem for other nodes to handle correctly without packet loss. This is especially a problem when TCP ACKs are transmitted on the affected (RLC retransmission) link. The reason is that TCP ACKs when received at the server will result in a TCP DATA transmission from the server as long as the server has more data to send. Normally the TCP DATA burst volume is ˜70 times larger in size than the TCP ACK burst volume, but for higher rates it has been seen TCP DATA bursts >1000 times than the TCP ACK bursts. The problem also increases with the increased line rates (e.g. when Iu and Iub support is increased from 1 Gbps to 10 Gbps).
For example, an RLC retransmission in WCDMA may take 200 ms. For a dual carrier capable connection this could mean that a corresponding TCP data burst can be up to 1 MB of data. This is a problem for many nodes to handle without packet loss.